In a conventional extrusion mold such as a hollow pipe molding apparatus thermoplastic material is extruded from a die into a pipe forming tunnel. The material initially in a molten state is cooled as it moves along the tunnel to provide a shaped pipe wall which emerges from the molding apparatus. The faster the material can be cooled, the higher the production rate.
If the material is only cooled from the outside of the molding apparatus, this creates very detrimental stress on the pipe. Therefore, there have been developments made with respect to cooling the material from inside the molding apparatus.
According to a known method, cooling air is drawn in from the upstream end of the mold through the die head and forced to move downstream with and cool the thermoplastic material. This method does however have some drawbacks. The air at its coldest temperature runs through and detrimentally cools the die and die tooling itself. Furthermore, by the time the cooling air reaches the mold tunnel where it is desirable to have the greatest effect on the pipe wall forming material it has lost much of its cooling capacity to the upstream tooling.
In a conventional internally cooled pipe mold used in forming single wall pipe, the pipe wall as it is released from the downstream end of the mold tends to reheat because the supposed cooling air has given up much of its ability to cool at the upstream end in the area of the die tooling. This leaves heat trapped in the pipe wall and reduces output speeds from the mold. The pipe itself when leaving the mold at an overly warm temperature coils rather than being straight as is desired. Once the pipe has coiled, the memory of the material holds the non-wanted coiled shape. Furthermore, it is very difficult to perforate the pipe wall when the thermoplastic material is warm rather than having been properly cooled and set.